1. Statement of the Technical Field
The invention concerns thermal energy cycles, and more particularly systems and methods for merging thermal energy cycles including multi-pass energy recirculation techniques which enable normally rejected thermal energy to be re-used in the cycle, repeatedly.
2. Description of the Related Art
Heat engines use energy provided in the form of heat to perform mechanical work, and exhaust a portion of the applied heat which cannot be used to perform work. This conversion of heat energy to mechanical work is performed by taking advantage of a temperature differential that exists between a hot “source” and a cold “sink.” Heat engines can be modeled on various different well known thermodynamic processes or cycles. Examples of typical thermal cycles are the Brayton Cycle, the Rankine Cycle and Refrigeration Cycles.
A combined cycle is an assembly of two or more of these processes that convert heat into mechanical energy, by combining the thermodynamic cycles. The exhaust of one heat engine associated with a first cycle is used to provide the heat source that is used in a second cycle. For example, an open Brayton cycle is commonly combined with a Rankine cycle to form a combined cycle for power plant applications. The open Brayton cycle is typically implemented as a turbine burning a fuel, and the exhaust from this combustion process is used as the heat source in the Rankine cycle. In such a scenario, the Rankine cycle is referred to as a bottoming cycle because it uses some waste heat from the Brayton cycle to perform useful work. When using high temperature sources of heat (e.g. 2000° F.), a combined open Brayton cycle with a Rankine bottoming cycle can ideally be expected to provide an energy conversion efficiency as high as 60%. In the case of low temperature heat sources (e.g. 700° F.) conversion efficiencies are much lower, traditionally below about 35%.